1. Field of the Invention
The present invention relates to a toner, developer, container for a developer and image forming process which are used for developing a latent electrostatic image on a photoconductor to form a visible image in electrophotographic apparatus and electrostatic recording apparatus.
2. Description of the Related Art
In electrophotography, electrostatic recording, and electrostatic printing, a developer is, for example, applied to an latent electrostatic image bearing member such as a photoconductor, so as to dispose the developer onto a latent electrostatic image formed on the latent electrostatic image bearing member in a developing step, the developer disposed on the image is transferred to a recording medium such as a recording paper in a transferring step, thereafter the transferred developer is fixed on the paper in a fixing step. Such developers used for developing the latent electrostatic image formed on the latent electrostatic image bearing member generally include two-component developers comprising a carrier and a toner, and one-component developers such as magnetic toner and non-magnetic toners, which do not require a carrier.
Conventional dry toners for use in electrophotography, electrostatic recording or electrostatic printing are formed by melting and kneading a binder resin such as a styrenic resin or a polyester, a colorant, and other components, then pulverizing the kneaded substance. Although improvement of toners has been attempted by miniaturize a diameter of toner particle in order to obtain high quality images, uniform particle shape cannot be obtained by ordinary manufacturing methods of kneading and pulverization. Moreover, the toner is still pulverized so that excessively toner particles are generated, in a course of mixing with carrier in a developing member of the apparatus, or, by a contact stress between a development roller, and a toner applying roller, a layer thickness controlling blade, or a friction charging blade. These lead to deterioration of image quality. In addition, a superplasticizer embedded in the surface of toner also leads to deterioration of image quality. Further, fluidity of the toner particles is insufficient because of their shapes, and thus a large amount of the superplasticizer is required or a packing fraction of the toner into a toner vessel becomes low. These factors inhibit miniaturization of apparatuses. Advantages of such dry toners having a small particle diameter are not effectively utilized. Toner particles prepared by pulverization have a lower limit of their particle diameter, and those having a further lower particle diameter cannot be obtained by this technique. In addition, such pulverized toner particles have irregular shapes, are not transferred satisfactorily and thereby invite image omission and an increased amount of toner to make up therefor.
Accordingly, a strong demand has arisen to yield high quality images which do not have any missing part and to reduce running cost by further improving transfer efficiency leading to a reduction in toner consumption. If transfer efficiency is remarkably excellent, a cleaning unit, which removes remained toner on a photoconductor or a transfer after transferring, can be omitted from an apparatus. Therefore, the apparatus can be miniaturized and low cost thereof can be achieved together with having a merit of reducing a waste toner. Hence, various methods for manufacturing a spherical toner have been suggested in order to overcome the defects caused by a non-uniformly shaped toner.
However, such spherical toner particles cannot significantly be removed by using a cleaning device such as cleaning blade or brush for removing residual toner from a photoconductor or a transfer medium, thus inviting cleaning failure. The spherical toner particles have their surfaces entirely exposed to surroundings and are thereby susceptible to the contact with a carrier or a charger such as charger blade. An external additive on their surface and a charge control agent located at the outermost surface are often embedded into the toner particles, and the flowability of the toner rapidly decreases, thus deteriorating durability of the toner. Such a dry toner is developed and transferred to a transfer member such as paper and is then fixed by heating and fusing with the use of a heat roll. If the heat roll temperature is excessively high, the toner is excessively fused and thereby adheres to the heat roll (hot offset). If the heat roll temperature is excessively low, the toner is not sufficiently fused, thus inviting insufficient image fixing. For saving energy and for downsizing apparatus, demands have been made on toners having a higher hot offset occurring temperature (higher hot offset resistance) and a lower image-fixing temperature (better low-temperature image-fixing properties). In addition, the toners must have high-temperature storage stability by which they are not blocked in ambient temperature during storage and in apparatus.
As a possible solution to solve these problems, Japanese Patent Application Laid-Open (JP-A) No. 07-152202 proposes a “polymer dissolving-suspending method” accompanying with volume shrinkage. In this method, a toner material is dispersed or dissolved in a volatile solvent such as a low-boiling organic solvent, the dispersion or solution is emulsified in an aqueous medium comprising a dispersing agent to form droplets, and the volatile solvent is removed. In the last process, the droplets undergo volume shrinkage. When a dispersing agent comprising solid particles that are insoluble in the aqueous medium is used, the resulting particles have irregular shapes. When a solid content in the solvent is increased to increase productivity, the disperse phase becomes viscous, and the resulting particles have large particle diameters with a broad distribution. In contrast, when the molecular weight of the resin is decreased to thereby decrease the viscosity of the disperse phase, satisfactory image-fixing properties such as hot offset resistance are not obtained.
JP-A No. 11-149179 proposes an improvement in image-fixing properties, in which a low molecular weight resin is used in the polymer dissolving-suspending method to decrease the viscosity of the disperse phase and facilitate emulsification, and the polymerization is performed within particles. This technique, however, does not improve transfer ability and cleaning ability by controlling the shape of particles.
To support for image-fixing at low temperatures, the use of a polyester resin having excellent low-temperature image-fixing properties and relatively good high-temperature storage stability has been proposed instead of styrene-acrylic resins conventionally used (JP-A No. 60-90344, No. 64-15755, No. 02-82267, No. 03-229264, No. 03-41470 and No. 11-305486). To improve low-temperature image-fixing properties, a specific non-olefinic crystalline polymer that can be fused sharply at its glass transition point is added to the binder (JP-A No. 62-63940). However, this technique does not teach an optimized molecular structure and molecular weight of the polymer. Separately, Japanese Patent (JP-B) No. 2931899 and JP-A No. 2001-222138 disclose the use of a crystalline polyester that can be fused sharply. However, the crystalline polyester in the toner disclosed in JP-B No. 2931899 has a low acid value and hydroxyl value of 5 or less and 20 or less, respectively, has low affinity for paper and thereby fails to provide sufficient low-temperature image-fixing properties. In addition, the molecular structure and molecular weight of the crystalline polyester are not optimized, and the microdomain structure in the toner for exhibiting the sharp melt ability of the crystalline polyester is not disclosed. This technique thus fails to provide sufficient low-temperature image-fixing properties. JP-A No. 2001-222138 also fails to disclose the microdomain structure in the toner for exhibiting the sharp melt ability of the crystalline polyester, thus failing to provide sufficient low-temperature image-fixing properties.
In image-fixing by contact heating, the surface of a heater is brought into contact with a fused toner under pressure (under a load), and the fused toner having a decreased viscoelasticity breaks when peeled off from the roller, and part of the toner image adheres to the surface of the heater and is then transferred to the image upon another contact, thus inviting hot offset. To avoid the hot offset, JP-A No. 09-251217 proposes a binder resin comprising a polyester resin comprising a novolak phenol resin and having satisfactory low-temperature image-fixing properties and hot offset resistance at high temperatures. JP-B No. 02675948 and No. 03128907, JP-A No. 07-333904, and JP-B No. 08-12475 each propose a toner comprising a specific polyester resin as a color toner having excellent hot off-set resistance. These toners, however, cannot produce high-quality sharp images under such conditions as to provide sufficient hot off-set resistance.